In mobile phone technology it is quite frequent that surrounding noises in the vicinity of the ear of a listener interfere with the acoustic reception at the ear and hence the listening comprehension. This is why there is an increased need for so-called active noise suppression systems which are also referred to as ANC systems, active noise cancellation systems. In such systems, a pre-definable frequency band of an interfering surrounding noise in the region of the loudspeaker or at the ear of a listener is suppressed.
Document GB 2449083 A, for instance, shows an ANC system with a feed forward control.
Instead of a feed forward control, feedback control systems are also possible as an alternative, as they may be used in headphones, for instance. With a feedback ANC system of this type, the microphone is in front of the loudspeaker. In the ideal case, the loudspeaker reproduces a microphone signal phase-shifted by 180°. In doing so, an external interfering noise which is picked up by the microphone is simultaneously superimposed with its own but inverted signal provided by the loudspeaker. This results in destructive interference of the two signals so that, in the ideal case, the result is 0 and the listener does not perceive any surrounding noise. The adaptation of this control circuit to the transfer function between loudspeaker and microphone, in other words the so-called open loop response, is to be taken into consideration. This open loop response depends in particular on the connection of the control system to the user. This connection involves in particular the force with which the system is pressed against the ear and its distance thereto. Usually, this is no problem in case of headphones. When putting on or taking off a headphone, for instance, the open loop response indeed changes, but not in a way which may result in instabilities, in particular oscillations in the control circuit of the feedback ANC. To give an example, ear pads and a relatively large volume within the headphone shells are responsible factors.
With mobile phones, however, these conditions do not exist as ear pads are not necessary. Moreover, a defined distance between the loudspeaker of the mobile phone and the ear of the user does usually not exist. Rather, the distance from the loudspeaker or the microphone of the mobile phone to the ear of the user is individually different to a high degree.
An adaptive filtering process, for instance in the context of a digital filtering process, of the connection of the system to the user and hence of the resulting time-variant open loop response of the system could basically follow. Through this measure, instabilities, in particular undesired whistling noises with high sound pressures, could be avoided. However, it appears that these control systems including an adaptive, digital filtering process are very elaborate in their implementation and in addition have further disadvantages such as phase shifts in the control system, in particular due to the propagation effects in the required analog/digital and digital/analog conversion. These delay times and phase shifts may, in turn, result in undesired instabilities of the control system.